2. Electric current is the rate of flow of charges
I=
Q
t
I=Electric current ; Q=charge ;t=time
Electric current is measured in amperes
1Ampere= 1coulomb/1second; A=C/S
One ampere is the current through a wire, if
charge flows at the rate of 1coulomb/second
Charge of an electron=1.6× 10¯19 c
Icoulomb= charge of 6.25×1018 coulombs
4. The drift velocity is the average velocity that a
particle, such as an electron, attains due to an
electric field.
V=
I
nAe
Where ,v=drift velocity I= current
n= free electrons per m³
A= cross sectional area
e= charge of electron
5. The potential difference between two points is the
electrical potential energy transferred to other forms,
per coulomb of charge that passes between the two
points.
Potential difference is measured in volts
Define 1 volt
V=W/Q; v=1volt if W=1joule and Q=1coulomb
The p.d between two points is 1volt if 1joule of
electrical energy is transferred when 1 coulomb of
charge flows between the two points
9. RESISTANCE
RESISTANCE is the opposition to the flow of
electrons
Tungsten filament has more resistance .But
copper wire has less resistance. Why?
The electrons make many collisions with the
tungsten ions as they move through the
filament. But electrons move more easily
through Cu wire as they make fewer collisions
10. Ohm’s Law
The current (I) flowing through a wire is directly
proportional to the p.d between its two ends if
the temperature remains constant.
11. Q)Define resistance
The resistance of a conductor is the ratio of the
potential difference applied across it to the
current passing through it.
Unit of resistance- Ohms
Q)Define 1 ohm
R=V/I ;R=1ohm if V=1volt and I= 1Ampere
The resistance of a wire is 1ohm if a current of
1Ampere flows through it when the p.d
between its two ends is 1volt.
14. CURRENT-VOLTAGE CHARACTERISTICS
• Graph is a straight line
• I is directly proportional to V.
• V/I is a constant
• The wire has constant resistance
15. FILAMENT LAMP
• V is not proportional to I
• V/I is not a constant but
increases with current.
• V/I=R increases with current
• As the current rises the tungsten
filament gets hotter and
resistance of the lamp rises
• The graph is symmetrical for both
positive and negative voltages
16. DIODE
• Diode conducts only when it
is forward biased.
• When reversed biased, there
is no current
17. • In the forward direction the
current increases very rapidly
• The forward biased diode has
very low resistance
• There is no current when the
voltage is applied in the
reverse direction.
• Reverse diode has very high
resistance.
18. EFFECT OF TEMPERATURE ON RESISTANCE
1. METALS : As temperature rises, the resistance of metals
increases because the ions vibrate faster with greater amplitude
and it is more difficult for electrons to pass through the lattice.
Metals have +ve temperature coefficient
2. INSULATOR: Resistance decreases with temperature.
insulators have few free electrons available for conduction at room
temperature. At high temperatures some electrons gain enough
Energy to escape from their atoms and the insulator is able to
Conduct.
They have –ve temperature coefficient
19. 3. SEMICONDUCTORS: Resistance decreases with
Increase in temperature. At low temperatures, semiconductors are
Poor conductors. As temperature rises, more and more electrons
Break free to make it a better conductor.
It has –ve temperature coefficient.
Eg: Thermistor
20. RESISTIVITY
1. R α l
R α 1/A
=> R α l/A
=> R = ℓ l/A
Where ℓ is a constant . This is equal to the resistivity of the
Material
Unit: Ohm meter (Ωm)
21. ℓ = R x A/l
Resistivity is defined as numerically equal to the
resistance of a sample of the material of unit length
and unit area of cross-section.
